Freya was an early warning radar deployed by Germany during World War II; it was named after the Norse Goddess Freyja. During the war, over a thousand stations were built. A naval version operating on a slightly different wavelength was also developed as the Seetakt.

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First tests of what would become the "Freya" were conducted in early 1937, with initial delivery of an operational radar to the Kriegsmarine in 1938 by the GEMA company. Freya supported an early version of Identification friend or foe (IFF). Aircraft equipped with the FuG 25a "Erstling" could be successfully queried across ranges of over 100 km.

The "AN" version gained a switchable phasing line for the antenna. Switching in the phasing line led to a phase displacement of the antenna's radiation pattern and with that, a "squinting" to the left or right. This enabled the system in effect to switch from the rather broad "scanning for maxima" to narrow lobe switching. In that mode, a skilled operator could achieve an angular resolution of 0.1°.

The Freya radar was more advanced than its British counterpart, Chain Home. Freya operated on a 1.2 m (3.9 ft) wavelength (250 MHz) while Chain Home used 12 m. This allowed Freya to use a much smaller antenna system, one that was easier to rotate, move and position. It also offered higher resolution, allowing it to detect smaller targets. Because of its complex design, only eight Freya stations were operational when the war started, resulting in large gaps between the covered areas. The British Chain Home radar, although less advanced and more prone to errors, was simpler, which meant that the complete Chain Home network was in place in time for the Battle of Britain.

Freya was often used in concert with the primary German gun laying radar, Würzburg Riese ("Würzburg Giant"); the Freya finding targets at long distances and then "handing them off" to the shorter-ranged Würzburgs for tracking.

Later in the war, Freya operated in the band from 2.5 to 2.3 m (8.2 to 7.5 ft) (120 to 130 MHz), with a pulse width of three microseconds, a peak power output of 15 to 20 kW, and a pulse repetition frequency of 500 Hz. It had a maximum range of only 160 kilometers (99 mi), making it inferior to Chain Home, nor could it accurately determine altitude, but it was a fully steerable and semi-mobile system.

Freya was first successfully used on December 18, 1939 when two stations detected an approaching daytime raid by 22 RAFVickers Wellington bombers at a range of 113 km and guided fighter planes toward them via radio.[1] Only half of the Wellingtons returned to Britain undamaged, but the German fighters only reached the bomber after they had made their bombing run on ships in harbor. Three German fighters had been shot down and a further 12 had received damage in the battle. The performance of Freya left the Luftwaffe so impressed that by the Spring of 1940, eleven Freya stations were installed to guard Germany's western border.[2] After the invasion of France in 1940, additional Freya stations were built along the Atlantic coast. When Britain started its bombing raids, Hermann Göring ordered colonel (later General) Josef Kammhuber to install an efficient air defense. This led to the so-called Kammhuber Line into which then more Freya stations were integrated. In the later course of the war, Freya devices turned out to be vulnerable against chaff, along with other countermeasures, which meant they could still be used for early warning but no longer for guiding fighter planes. British bombing raids could also be organized such that the Kammhuber Line could be overwhelmed in massed raids.

One of the first to give British intelligence any details about the Freya Radar was a young Danish Flight Lieutenant, Thomas Sneum, who, at great risk to his life, photographed radar installations on the Danish island of Fanø in 1941. He brought the negatives to England in a dramatic flight which is fictionalized in Ken Follett's novel Hornet Flight. Sneum's deed is also mentioned in R. V. Jones'sMost Secret War as a 'most gallant exploit'.

FuMG 401: For experiments with beam reflection on the ground, leading to a change in elevation angle, a Freya antenna array was installed on a wooden support so it could slide up and down. This enabled Freya to detect the target's altitude without the aid of other devices (such as Würzburg radars).

FuMG 41: In an attempt to increase the range without changing the transmitter, several Freya antenna arrays were switched together. These installations, called "Wassermann" (German: Aquarius), not only had greater range but were also more accurate.

To counter Freya, the British used equipment called 'Moonshine'. Carried by Boulton Paul Defiant aircraft of the Special Duties Flight (later No. 515 Squadron RAF), a single set retransmitted a portion of the Freya signal amplifying the apparent return. Eight planes with 'Moonshine' could mimic a force of 100 bombers.[3] A second countermeasures system, "Mandrel" was a noise jammer which overwhelmed the signals from Freya. Individual aircraft were sent to orbit fixed positions 50 miles (80 km) off the enemy coast. By using nine airplanes, a 200-mile (320 km) gap could be knocked into the German's radar coverage, while further jammers were carried in the bomber stream to counter the inland Freya network.[4]